Laser shock peening or laser shock processing, as it is also referred to, is a process for improving the properties of a metallic material from which a workpiece is fabricated. As with any peening process, for example shot peening, it does this by producing a region of deep compressive residual stresses in the material at the surface of a workpiece. The region of such compressive residual stresses in the surface of a workpiece provides a protection layer which beneficially increases the surface material's resistance to surface-related failures such as fatigue, fretting and stress corrosion cracking. A difference, and advantage of, laser shock peening is that this region is considerably deeper than that produced by other peening methods.
The process of laser shock peening involves coating the surface of a workpiece to be treated with a coating that is opaque to a laser to provide an opaque overlay. A transparent overlay, typically a water film, is provided on top of the opaque overlay. A high power pulsed laser beam is then directed onto, and scanned across, the surface of the workpiece. The laser beam passes through the transparent overlay and causes instantaneous ablation or vaporisation of the opaque overlay. The vapour is trapped by the transparent overlay and the explosive force and high pressure shock wave produced is directed into the surface of the workpiece. This results in a localised compressive force on a portion of the surface which produces the beneficial region of deep compressive residual stresses in a surface of a workpiece. Such methods are described in U.S. Pat. No. 3,850,698.
To provide the required degree of compressive residual stress in the surface of the workpiece the above process may be repeated. For example, to provide a suitable protection layer in the surface of workpieces which are to be used in gas turbine engines the process may be repeated up to three times.
The opaque overlay coating is conventionally a black paint that is painted onto the surface of the workpiece and dried. The painted workpiece is then transferred to a separate machine where the transparent overlay is applied and the laser beam is directed at the workpiece.
The painting and drying steps are time consuming and complex. In particular if repeated laser shock peening is required, to produce the desired level of compressive residual stress, then the workpiece has to be repeatedly removed from a laser shock peening machine, painted, dried and then refitted into the machine.
The problems with painting a workpiece within the laser shock peening process are described in U.S. Pat. No. 5,674,329 along with an alternative method. As described in that patent the painting and drying steps are replaced with a less time consuming taping step. The tape which is applied to a workpiece is self adhesive and has an ablative medium which forms the required opaque overlay on the workpiece. Whilst eliminating the time consuming painting and drying steps this method is not ideal. Applying tape to a complex component may in itself be difficult and time consuming and the cost of a the tape may be significant. Furthermore this method described does not address the problem of having to remove the workpiece from the laser shock peening machine in order to reapply the opaque overlay (either paint or tape) during repetition of the laser shock peening process.